1. Field of the Invention
This invention relates to the field of gas chromatography. More particularly, this invention relates to a micro-gas chromatograph device that is formed from multiple layers of green-sheet and also relates to methods for making such devices.
2. Description of Related Art
Gas chromatography is a well-established analytical technique that is commonly used for the separation and detection of the various chemical components present in gases and low boiling point liquids. The technique is widely used in organic chemistry research, pharmaceutical development, and forensic specimen analysis. A gas chromatography system typically has five major components: (1) a carrier gas; (2) a sample injector; (3) a gas chromatography column; (4) a detector; and (5) a data processing system. The carrier gas, also referred to as the mobile phase, is a high-purity and relatively inert gas, such as helium. The carrier gas flows through the column throughout the separation process. The sample injector introduces a precise and, typically, very small volume of the sample, in gaseous form, into the flow of carrier gas into the column. The gaseous sample typically includes a number of different chemical components that are intended to be separated by the gas chromatograph. To effect this separation, the inside of the column is coated with a stationary phase that adsorbs the different chemical components in the sample to differing degrees. These differences in adsorption cause differing propagation delays for the chemical components as they travel down the column, thereby effecting a physical separation of the sample into its chemical components. The detector is located after the column and serves to detect the various chemical components in the sample as they emerge from the column at different times. The data processing system reads the detector and is typically able to store, process, and record the results.
Conventional gas chromatography systems are bench top instruments that are designed for use in a laboratory setting. However, in many instances, it is desirable to have a portable gas chromatograph that can be used outside of the laboratory, such as where the samples are collected. Portable gas chromatographs have potential application for leak detection, environmental screening, monitoring the volatile organic chemical content of waste water, and in the detection and analysis of vent gases, land fill gases, and natural gas.
One of the most significant barriers to making a portable gas chromatograph device is that the separation efficiency of the device is directly proportional to the length of the column. Currently, a few portable gas chromatography systems are available, but they are only suited for the detection of certain specific substances. In recent years, efforts have been made to fabricate the column and detector using newly developed micromachining techniques in order to provide miniaturized gas chromatography systems that are portable and that can analyze multiple substances.
Such micro-gas chromatograph devices are most commonly fabricated from silicon substrates. However, such substrates have a number of disadvantages. For example, a micro-gas chromatograph column has been fabricated by etching an interlocking spiral channel about 10 microns deep and 300 microns wide in a silicon wafer. See Reston, et al., xe2x80x9cSilicon-Micromachined Gas Chromatography System Used to Separate and Detect Ammonia and Nitrogen Dioxide,xe2x80x9d J. Microelectromechanical Systems, 3:134-146 (1994). The top surface of the column was defined by a borosilicate glass plate anodically bonded to the silicon wafer. Because the bond frequently failed along the edges, presumably because of the mismatch in thermal expansion coefficients of the two materials, the column was restricted to an area in the center of the wafer about 3.8 cm in diameter. Accordingly, the anodic bonding process used with silicon substrates serves to limit the length and, thus, the separation efficiency of the column. Another limitation on the length of the column in the Reston device is that it lies all in one plane, namely, the interface of the silicon and glass layers. Still another disadvantage with this approach is that, because the column is defined by dissimilar materials, thermal gradients can develop that further decrease the column""s separation efficiency.
Goedert, U.S. Pat. No. 4,935,040 discloses a micro-gas chromatograph device that is made up of multiple layers. Several planar column sections are defined by the interfaces between pairs of layers, and the planar column sections are connected in series to increase the available column length. The layers alternate between silicon and glass wafers that are joined together by anodic bonding. Alternatively, the layers may be silicon, with bonding effected by a thin layer of silica between. By using multiple layers, the Goedert device is able to provide a longer column. However, anodically bonding multiple layers is difficult to achieve reliably.
In a first principal aspect, the present invention provides a multilayered micro-gas chromatograph device for analyzing an analyte gas that includes a plurality of chemical components. The multilayered micro-gas chromatograph device comprises a substantially monolithic structure having a micro-gas chromatograph column defined therein. The substantially monolithic structure is formed from a plurality of green-sheet layers sintered together, wherein the green-sheet layers include particles selected from the group consisting of ceramic particles, glass particles, and glass-ceramic particles. The micro-gas chromatograph column has an inlet port for receiving the analyte gas and an outlet port for releasing the analyte gas. A stationary phase for differentially adsorbing chemical components in the analyte gas is disposed in a portion of the micro-gas chromatograph column.
In a second principal aspect, the present invention provides a micro-gas chromatography system comprising a supply of a carrier gas, a sample injection valve, a micro-gas chromatograph column, and a detector. The sample injection valve is connected to the supply and injects a sample gas into the carrier gas to provide an analyte gas. The micro-gas chromatograph column separates the analyte gas into a plurality of chemical components. It has an inlet port and an outlet port, with the inlet port connected to the sample injection valve to receive the analyte gas. The micro-gas chromatography column is defined in a substantially monolithic structure that is formed from a plurality of green-sheet layers sintered together. The green-sheet layers include particles selected from the group consisting of ceramic particles, glass particles, and glass-ceramic particles. The detector is connected to the outlet port and detects the plurality of chemical components separated by the micro-gas chromatograph column.
In a third principal aspect, the present invention provides a method for making a multilayered micro-gas chromatograph device. A plurality of green-sheet layers are textured in a predetermined pattern to define a micro-gas chromatograph column. The green-sheet layers include particles selected from the group consisting of ceramic particles, glass particles, and glass-ceramic particles. A thick-film paste is applied to at least a portion of the predetermined pattern in the green-sheet layers. The green-sheet layers are sintered together at a predetermined temperature for a predetermined amount of time to form a substantially monolithic structure having a micro-gas chromatograph column defined therein, with a porous plug, formed from the thick-film paste, disposed in the micro-gas chromatograph column.